1. Field of the Invention
The present invention relates to an optical information recording/reproducing apparatus having an expanded range within which a track error signal can be detected.
2. Related Art Statement
Recently, optical information recording/reproducing apparatuses capable of serving as large-capacity recording apparatuses have attracted public attention as the information industry advances. These optical information recording/reproducing apparatuses are exemplified by an optical card apparatus for recording/reproducing information which uses an optical card as the recording medium. Although data cannot be reloaded to optical cards as it can to reloadable optical disks, optical cards are expected to be widely applied for bankbooks, handy maps, prepaid cards or the like for use in shopping and so on, since the optical card has a recording capacity which is a thousand to ten thousand times larger than the recording capacity of the magnetic card and has a large storage capacity of 1 to 2M bytes. Furthermore, the fact that data cannot be reloaded into the optical card enables it to be applied to an application such as a health care card which does not permit data to be rewritten.
As shown in FIG. 1, an optical card comprises ID portions 15a and 15b on which information such as a track address is recorded and a data recording portion 16 for recording information. The data recording portion 16 has a plurality of guide tracks 11 and tracks 12 formed between the guide tracks 11, the guide tracks 11 being arranged to guide the light spot in the direction of the tracks 12. Each of the tracks 12 has data pits 13 for recording/reproducing information.
When information recorded/reproduced in the data pits 13 of the track 12 is recorded/reproduced by means of light spots, accurate tracking servo must be performed in order that the light spots accurately follow the tracks 12. The tracking servo is ordinarily performed in such a manner that a tracking error signal indicating the deviation of a light spot from the central portion of the track is generated, and means for converging the light spot onto the medium is driven in the direction of the track in accordance with the tracking error signal.
FIG. 2 illustrates the conventional structure of an optical system for detecting a track error signal.
Light beams emitted from a light emitting device 2 are shaped into parallel light beams by a collimeter lens 3, diffracted light beams are formed by a diffraction grating 50, and the light beams are focused on the surface of the optical card 1 by an objective lens 4.
The focused light beams are reflected by the card 1, these reflected light beams are again reflected by a mirror 5, and the light beams are made to be incident on a detector 52 by a detection-system lens 51.
FIG. 3 is an enlarged view of some optical beams applied to a focal point on the surface of an optical card 1. A beam 53 diffracted by the diffraction grating 50 called a "main beam" is used to record/reproduce data in the data pit 13 and to generate a focus error signal for the purpose of controlling focusing, while beams 54 and 55 called "sub-beams" are respectively disposed in such a way that half of each overlaps the guide track 11, and are used to generate track error signals.
FIG. 4 illustrates beams reflected by the optical card 1, where beams 53a, 54a and 55a correspond to beams 53, 54 and 55 shown in FIG. 3.
The detector 52 is further divided into sections 52a, 52b, 52c and 52d. If the beam on the optical card 1 deviates from the focused state, the beam 53a is moved in a direction perpendicular to the division lines of the detectors 52a and 52b. Therefore, a focus error signal denoting the deviation from the focus position can be obtained by calculating the difference between the output values of detectors 52a and 52b. By using the focus error signal to drive an objective lens 10 so that it approaches or moves away from the card, focusing control is performed for the purpose of always maintaining the focused state.
When the beams 53, 54 and 55 are moved in a direction perpendicular to the tracks 12, the degree to which the beams 54 and 55 overlap the guide track is changed. Therefore, by calculating the difference between the output values of the detector 52c and 52d, a track error signal indicating the deviation of the beam from the central portion of the track can be obtained. By using the track error signal to drive the objective lens 10 in a direction perpendicular to the tracks, tracking control is performed for the purpose of always maintaining the beam at the central portion of the track.
FIG. 5 illustrates the track error signal when the light spot is displaced in a transverse direction with respect to the track, the track error signal having a waveform the period of which is the width of the track.
However, the method of detecting the track error signal adapted to the aforesaid conventional apparatus encounters the following problems:
(1) The range in which the track error signal can be detected is narrow and therefore the apparatus cannot satisfactorily withstand vibrations and shock. PA1 (2) The apparatus can easily be affected by dust or a defect such as a flaw in the card and therefore the apparatus cannot perform satisfactorily if such conditions are present.
As for problem (1), the range in which the track error signal can be detected is determined on the basis of the diameter of the beam spot and the width of the track in the aforesaid conventional method. Therefore, the detection range is ordinarily limited to the width of one track (2 to 3 .mu.m).
Therefore, the track error signal can be swung considerably if an acceleration acts on the objective lens 10 due to external vibrations or shock, so as to exceed the linear portion of the track error signal which can be used as the signal. As a result, the tracking servo becomes unstable and deviates to another track.
As for problem (2), the track error signal can be swung excessively if either of the beams 54 and 55 shown in FIG. 3 has a defect.
Since a beam usually has a diameter of about 2 to 3 .mu.m, the track error signal is critically affected by even a small defect. In this case, also the tracking servo becomes unstable and deviates to another track.